The present invention relates to a measurement system. In particular, the present invention relates to a capacitance-to-digital modulator for use with a capacitive pressure sensor.
A field transmitter is a device that is used to monitor the operation of an industrial process. The field transmitter includes a transducer that responds to a measured process variable with a sensing element and converts the variable to a standardized transmission signal that is a function of the measured variable. The term “process variable” refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, and pH.
One such transmitter is described in U.S. Pat. No. 6,295,875 by Roger L. Frick and David A. Broden. This transmitter employs a capacitive sensor having a deflectable sensing diaphragm and three or more capacitor electrodes which form separate capacitive sensing elements with the diaphragm. Two of the capacitor elements are primary sensing capacitors that are arranged differentially so that the capacitances of the primary sensing capacitors charge oppositely in proportion to the process variable. The third and fourth capacitor elements are compensation capacitors that provide signals representing offset errors or hysteresis associated with the primary capacitors. As pressure is applied to one or both sides of the diaphragm, the diaphragm deflects. The deflection of the diaphragm can be detected by measuring a change in a ratio of electrical capacitance related to the deflection. This capacitance ratio is converted into a digital format using an analog-to-digital converter.
Another type of transmitter is described in U.S. Pat. Nos. 5,637,802 and 6,089,097 by Roger L. Frick, Bennett L. Louwagie and Adrian C. Toy. The transmitter described in these two patents uses two absolute pressure sensors to measure differential pressure as well as two absolute pressures, with high resolution on the differential pressure measurement.
One particularly advantageous form of analog-to-digital converter uses a sigma-delta (or delta-sigma) modulator. The use of sigma-delta modulators in transmitters is described in U.S. Pat. No. 5,083,091 by Roger L. Frick and John P. Schulte; U.S. Pat. No. 6,140,952 by Michael Gaboury; U.S. Pat. No. 6,509,746 by Rongtai Wang; and U.S. Pat. No. 6,516,672 by Rongtai Wang.
In a transmitter having a sigma-delta modulator acting as a capacitance-to-digital (CD) converter, an excitation circuit provides charge packets to the capacitive sensor elements. The sensor elements are charged by an amount based on the capacitance value of that capacitive element. The charges are transferred to an integrator/amplifier of the sigma-delta modulator to produce a one-bit binary output which is a function of a capacitance ratio.
The basic function of the CD modulator is to convert the capacitance ratio into a PCM (pulse code modulation) signal. For a CD modulator using sigma-delta architecture, the actual process involves converting a charge ratio into a PCM signal. Under normal operating conditions, since the charge is proportional to the capacitance, the charge ratio is equal to the capacitance ratio.
However, this equivalent relation is not true under certain abnormal operating conditions. Sensor failure mode conditions can occur in which the sensor is either disconnected from the CD converter or has failed due to a loss of physical integrity. The CD modulator may continue operating as usual without providing warning signals, even though the pressure readings no longer represent the true pressure under measurement.